The invention relates to a porous, gas permeable layer substructure for a thin, gas tight layer. The layer substructure is provided for use as a functional component in high temperature fuel cells. The invention is also directed to an electrochemically active element for fuel cells and a method for the manufacture of the layer substructure.
Electrode reactions for the production of an electric current are carried out on electrochemically active membranes in high temperature fuel cells: namely a reducing reaction at the anode in which water or carbon dioxide are produced from hydrogen and carbon monoxide of a first gas flow; and an oxidizing reaction at the cathode, in which ionic oxygen O2− is formed from molecular oxygen of a second gas flow while taking up electrons from a metallic conductor. The oxygen ions move through a solid electrolyte which separates the two electrodes in a gas tight manner and which is conductive for the oxygen ions at temperatures over 700° C. The reducing anode reaction with the oxygen ions takes place while discharging electrons to a further metallic conductor which makes a connection to an adjacent electrochemically active membrane or to a battery terminal. The electromechanically active membrane is a multi-layer system in whose manufacture each layer can be used as a carrier for an adjacent layer, and this adjacent layer can be produced by means of thin film technology, for example a thermal spray method or using a screen printing method.
A high temperature fuel cell whose solid electrolyte is formed as part of a multi-layer membrane in the form of a thin layer is known from EP-A-0 788 175. Such thin film electrolytes, which are 10-20 μm thick, can be applied to a porous carrier structure by means of a VPS method (vacuum plasma spray method). The carrier structure, which has to be permeable for the reactants, is made up of a base layer with large pores and a top layer with fine pores. It forms the substructure for the further layers of the electrochemically active element. The top layer can be made of a slurry and be rolled to form a thin film by means of a tape casting method, subsequently applied to the base layer and connected thereto by sintering.
In a preferred form, the carrier structure also has an electrochemical function as an electrode and an electric function as a good conductor in addition to its mechanical function as a carrier. The material for the cathode layer can be applied to the solid electrolyte layer by means of a screen printing method and using a slurry. This material is subsequently further converted into a functional layer by sintering.